In general, a method of implementing a white luminous element is to combine a blue light emitting diode and a yellow light phosphor capable of being excited by blue light, or to combine green and red light phosphors capable of being excited by blue light. The former case has limitation on implementation of color rendering, whereas the latter case can implement a high color rendering index equal to or greater than 85.
As for the structure of a white luminous element, it has a GaInN semiconductor structure, and comprises a combination of a blue light emitting diode having a maximum light emission wavelength of 430 to 470 nm and a phosphor capable of being excited by a portion of blue light to perform conversion into yellow light, or a combination of phosphors capable of being excited by a portion of blue light to perform conversion into green and red light. As for the composition of a green light phosphor, a thiogallate group excited by blue light to emit green to yellow light can be used. The typical composition of the thiogallate group is represented by (Ca,Sr,Ba)(Al,Ga,In)2S4:Eu(or Ce). In this group, SrGa2S4:Eu phosphor is a green light phosphor with higher light emission intensity. On the other hand, a red light phosphor is used as a light emission source of red spectrum in which SrS:Eu, (Sr,Ca)S:Eu, CaS:Eu and the like utilize blue light as an excitation source.
However, the (Sr,Ca)S:Eu red light phosphor has serious drawbacks in that its light emission intensity is not sufficiently high and its chemical stability against moisture is lowered. Further, since the spectrum of the (Sr,Ca)S:Eu red light phosphor has a broader half-width, it overlaps with the spectrum of green light when the (Sr,Ca)S:Eu red light phosphor is applied to a white backlight for an LCD. Consequently, it is difficult to implement green and red color coordinates, which is regulated by the National Television Standard Committee (NTSC), after filtration through a color filter. Even in U.S. Pat. Nos. 6,252,254, 6,521,915, 6,501,102, 6,680,569, 6,686,691, 6,783,700 and the like, which disclose red light phosphors and luminous elements using the same, it cannot be significantly expected to obtain chemical stability and higher efficiency due to the above reasons. Therefore, there is a drawback in that they cannot be applied to a general lighting apparatus and a backlight for an LCD.